It is well known in the art that brazing of aluminum parts can be performed utilizing fluxes based on alkali metal fluoroaluminates. Fluxes of this type are generally considered to be noncorrosive. See for example, U.S. Pat. No. 3,971,501 which applies a flux based on KAlF4 and K3AlF6, or U.S. Pat. No. 4,689,092 which applies a flux based on potassium fluoroaluminate and cesium fluoroaluminate. U.S. Pat. No. 6,949,300 discloses kinetic spraying onto metal substrates a brazing composition that comprises corrosion protector, brazing filler and/or non-corrosive flux.
EP-A-0 091231 discloses a flux comprising LiF in an amount of from 2% by weight to 7% by weight. The flux has a lowered melting point and is said to be suitable for brazing of aluminum alloys containing magnesium. In two examples, items were brazed with a flux containing K3AlF6, KAlF4 and Li3AlF6. The brazed items were subjected to a brine test and showed no corrosion after a treatment of 1000 hours.
In GB-A 2 224 751, a method of treating an aluminum workpiece is described. A treatment of the workpiece with a carbon oxide, e.g., during brazing, is provided. Hereby, the workpiece blackens. The formation of a black coating is improved when LiF is present in the flux.
JP 07-009123 discloses a flux with a melting point of 560° C. or less which is suitable for brazing of aluminum alloys containing magnesium. In two examples, fluxes with 3 and 10% by of LiF, respectively, were applied.
U.S. Pat. No. 5,802,752 discloses a method for open-air flame brazing (torch brazing). A flux is applied which contains a potassium fluoroaluminate flux and from 1 to 30% by weight, most preferably from 6 to 11% by weight, of cesium fluoride, lithium fluoride or both. The flux is well suited to braze aluminum alloys containing magnesium.
EP-A-0 347 106 discloses a method for treating aluminum workpieces by heating them in an oxidizing atmosphere to improve their corrosion resistance. In some of the examples, among other inorganic additives, LiCl, Li3AlF6 or LiF are added in an amount of 4.8% by weight. The effect is enhanced when additives such as alkali and alkaline earth metal salts are added to the flux.
EP-A-0 541 259 discloses a flux comprising KF—AlF3 and, in an amount of 0.75 to 16.5% by weight of LiF. The flux is obtained by melting appropriate amounts of KF, LiF and AlF3 in a crucible. The brazed parts, when immersed in water, provide a relatively low conductivity to the water.
If contacted for extended times with water or aqueous liquids, aluminum parts brazed with potassium fluoroaluminate based fluxes show signs of corrosion. This is disclosed by Bo Yang et al. in Journal of ASTM International, Vol. 3, Issue 10 (2006). The corrosion can be recognized by the appearance of turbidity in the water or liquid and seems, for example, to induce the formation of aluminum hydroxide.
This corrosion seems to be caused by fluoride ions which are leached from brazing residues if the brazed parts are in contact with water for extended periods of time, e.g., for at least one day or longer.
It is known that aluminum is always coated by aluminum oxide which prevents a good braze joint; the purpose of adding a flux is to clean the surface of the aluminum parts to be brazed, and notably, to remove the oxide layer. Often, potassium fluoroaluminate based fluxes are applied. Often, these fluxes consist of KAlF4 or of a mixture of KAlF4 and K2AlF5. The content of K3AlF6 in the flux should be low. During the brazing process, K3AlF6 is often formed, and these fluxes can be considered as precursor for K3AlF6. It is assumed that K2AlF5 disproportionate to KAlF4 and K3AlF6 according to the following equation:2K2AlF5KAlF4 and K3AlF6  (1)
Other fluxes, e.g., potassium fluorozincate, form potassium fluoroaluminates in situ:2Al+3KZnF33Zn+KAlF4+K2AlF5  (2)
The K2AlF5 formed will disproportionate according to equation (1) forming KAlF4 and K3AlF6. Thus, such fluxes are precursors of K2AlF5 and K3AlF6, and the overall equation is4Al+6KZnF36Zn+3KAlF4+K3AlF6  (2a)
K2SiF6 forms K2AlF5, and, see (3b), subsequently K3AlF6, too:4Al+3K2SiF63Si+2KAlF4+2K2AlF5  (3)4Al+3K2SiF63Si+3KAlF4+K3AlF6  (3b)The K2AlF5 formed will disproportionate, as described above, to form KAlF4 and K3AlF6. Also potassium fluorostannates are precursors of K2AlF5 and K3AlF6.
It was found that in prolonged contact with water, K3AlF6 releases KF which causes corrosion on the surface of the brazed aluminum parts.